how do i compile c# code to IL code instructions

Im making a thing that recompiles executeables with modifyed IL code and need to get the OPcode instructions (no classes, methods etc just the instructions) so that i can just add/replace it in the method and then recompile it.

is there someway i can compile this c# code:

int a = 5;
int b = 24;
Console.WriteLine("{0} * {1} = {2}",a,b,a*b);

to the il code:

.entrypoint
// Code size       38 (0x26)
.maxstack  5
.locals init ([0] int32 a,
         [1] int32 b)
IL_0000:  nop
IL_0001:  ldc.i4.5
IL_0002:  stloc.0
IL_0003:  ldc.i4.s   24
IL_0005:  stloc.1
IL_0006:  ldstr      "{0} * {1} = {2}"
IL_000b:  ldloc.0
IL_000c:  box        [mscorlib]System.Int32
IL_0011:  ldloc.1
IL_0012:  box        [mscorlib]System.Int32
IL_0017:  ldloc.0
IL_0018:  ldloc.1
IL_0019:  mul
IL_001a:  box        [mscorlib]System.Int32
IL_001f:  call       void [mscorlib]System.Console::WriteLine(string,
                                                              object,
                                                              object,
                                                              object)
IL_0024:  nop
IL_0025:  ret

so that i can replace it and reassemble it with ilassem?

Solution

If you're looking for a tool that can show you the IL, you can use LINQPad:

Select "C# Statements"
Type your code
Optional: by pressing the button in the lower right corner of the window, choose whether you want to see optimized IL or not
Press F5 or the green Execute button
Select the IL tab

The result for your code is:

IL_0000:  nop         
IL_0001:  ldc.i4.5    
IL_0002:  stloc.0     // a
IL_0003:  ldc.i4.s    18 
IL_0005:  stloc.1     // b
IL_0006:  ldstr       "{0} * {1} = {2}"
IL_000B:  ldloc.0     // a
IL_000C:  box         System.Int32
IL_0011:  ldloc.1     // b
IL_0012:  box         System.Int32
IL_0017:  ldloc.0     // a
IL_0018:  ldloc.1     // b
IL_0019:  mul         
IL_001A:  box         System.Int32
IL_001F:  call        System.Console.WriteLine
IL_0024:  nop         
IL_0025:  ret

Note that this is not code that can be directly compiled with ilasm, because it doesn't use the full syntax for types and members and also because it does not declare locals.